1. Field of the Invention
The present invention relates to an image capture apparatus and a method of controlling the same.
2. Description of the Related Art
An image capture apparatus such as a digital camera or a cellular phone with a camera is designed to set a gain for an output from an image sensor so as to exploit a range, of the input-output characteristics of the image sensor, which exhibits linearity (Japanese Patent Laid-Open No. 11-331689). This apparatus is provided with a linear conversion circuit (also called a gain circuit) which converts an output from the image sensor with a constant gain exhibiting linear input-output characteristics. The apparatus is also provided with a nonlinear conversion circuit (also called a gamma circuit) which converts the level of an image signal with a nonlinear input-output characteristic so as to control tone characteristics. For example, Japanese Patent Laid-Open No. 2006-157658 discloses a technique of performing tone correction by adjusting the input-output characteristics (also called gamma characteristics or gamma curve) of a nonlinear conversion circuit.
In order to ensure a dynamic range, the image sensor is sometimes driven with a constant sensitivity less than the set sensitivity to perform shooting operation regardless of the set sensitivity as in a case of driving the image sensor with a sensitivity less than the set shooting sensitivity. In this case, preparing a gamma curve for each sensitivity can compensate for the difference between the shooting sensitivity and the actual device driving sensitivity and exploit the input range of a gamma curve, thereby obtaining good tone gradation (Japanese Patent Laid-Open No. 2006-157658). FIG. 3 shows an example of a gamma curve corresponding to each shooting sensitivity.
However, preparing a gamma curve for each shooting sensitivity as shown in FIG. 3 will require a large storage capacity. The larger the number of shooting sensitivities which can be set, the more serious this problem will be. Some digital cameras are configured to set shooting sensitivities by an automatic exposure control function as well as allowing the user to explicitly set shooting sensitivities. In general, in many digital cameras, shooting sensitivities which can be set by the user are limited to values that change by 2 or ½ times like film speeds. However, shooting sensitivities which can be set by the automatic exposure control function can be values set in smaller steps. This is because the shooting sensitivities of a digital camera can be electrically controlled unlike film speeds.
In an image capture apparatus having an automatic exposure control function of deciding a shooting sensitivity, since the number of types of shooting sensitivities which can be set can be very large, the demerit of preparing a gamma curve for each shooting sensitivity further increases.